So what does this mean? Once you understand this information, you need to determine the minimum and maximum output voltage of the converter. This is simply the total number of all LED forward voltage drops plus the sense resistor voltage. Depending on the converter input voltage range, it can be determined whether the output voltage is always large, small, or in between.
Four common topologies for powering LEDs
Boost
As the name implies, the boost converter output is always greater than its input voltage and is usually the most efficient of the converters shown above. Benefits include clamped FET voltage (which minimizes vibration and noise) and low input ripple current (supports small input capacitors). Disadvantages are higher currents in inductors, FETs, and diodes (higher than the output), higher losses in the FET due to high voltage switching and higher currents, and ripple currents in the output capacitor. If the output current is much less than 1A, then these shortcomings are usually negligible.
Buck
A buck converter should be used when driving a small number of LEDs through a higher input voltage. It is typically capable of achieving extremely high efficiency with a minimum overall package size. Advantages include component stress strengths less than or equal to Vin maximum and less than or equal to Iout, and low output ripple current (supporting small output capacitors). The downside is that it has a pulsating input current and may vibrate at the FET switching voltage. A good practice is to simplify the control loop by connecting the output capacitor in parallel with the LED (not grounded).
Inverted buck-boost
Inverting buck-boost is a "between the two" topology option, meaning that the converter can be used as a buck or boost. Even if the output is negative, the LED doesn't really care, so you don't have to stop. Since the controller is referenced to –Vout, the current sense resistor feedback connection is similar to a standard buck topology. In fact, it is generally difficult to find the difference between it and the buck topology, because many connections are the same, but the circuit works differently. Benefits include the use of a buck (or synchronous buck) controller and buck-boost! Disadvantages are lower efficiency, greater current strength (similar to boost topology), and level shifting if controller activation is required.
SEPIC
The single-ended primary inductor converter (SEPIC) is a non-inverting buck-boost topology. The inductor can be coupled (like a transformer) or completely separate. Advantages include clamp switching operation similar to boost converters (ideal for low noise, high switching frequency) and low input ripple current (for small input capacitors). The disadvantages are the highest component usage, low efficiency, highest current stress, and the most difficult to understand complex control loops (but use at lower bandwidths is generally not a problem).
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